1. Field of the Invention
This invention relates to a photo-taking lens assembly in which a lens is automatically and manually movable, and in particular to a photo-taking lens assembly having an automatically and manually movable lens which is suitable for use with cameras capable of automatic focusing or the like.
2. Related Background Art
In a so-called TTL type automatic focusing apparatus wherein the in-focus state by light passed through a photo-taking lens is detected and a drive motor is controlled on the basis of the detection signal to thereby accomplish focus adjustment, focus detection may sometimes become impossible if the image of an object to be photographed is dark or too blurred and low in contrast. In such a state in which focus detection is impossible, it becomes impossible to automatically accomplish focus adjustment and therefore, various proposals for an automatic focusing apparatus also capable of manual focusing have heretofore been made and already are known. These known focusing apparatuses are divided broadly into the type as disclosed, for example, in Japanese Laid-Open patent application No. 73025/1980 wherein automatic focusing and manual focusing are selectively changed over by change-over means, and the type as disclosed, for example, in Japanese Laid-Open patent application No. 114008/1983 wherein a lens different from a movable lens for manual distance adjustment is moved to accomplish automatic focusing.
However, in the former prior art, the operation of using the change-over means has been cumbersome and quick change-over could not be accomplished. Also, even if the photographing conditions for the object to be photographed change during manual focusing to bring about a state in which automatic focusing is possible, as long as the photographer is not aware of it, focus adjustment must be effected manually, and this leads to a disadvantage that the function of a camera with an automatic focusing apparatus cannot be sufficiently performed. Also, in an apparatus wherein the range of movement of the movable lens for focusing is great, the time required for focusing becomes long or short depending on the initial position of the movable lens group and further, where the weight of the movable lens group is great, the speed of movement of the lens is low, and this has impaired quickness of photographing.
The latter known focusing apparatus has an advantage that automatic focusing and manual focusing can be accomplished without interfering with each other and the change-over device for automatic focusing and manual focusing is not required, while it suffers from a disadvantage that the construction of the optical system is complicated. In an apparatus wherein a teleconverter for automatic focusing is added to the interchangeable lens as in the embodiment of the aforementioned Japanese Laid-Open patent application No. 114008/1983, the teleconverter is designed to be versatilely used for various interchangeable lenses, and this leads to the following problems. That is, the above-mentioned teleconverter for automatic focusing has suffered from the following disadvantages:
(1) The combined F-number of the interchangeable lens and the teleconverter always becomes greater than the F-number of the interchangeable lens itself and the photo-taking lens is a substantially dark lens.
(2) If there are aberrations in the interchangeable lens, the aberrations tend to be amplified by the teleconverter and the composite imaging performance deteriorates.
(3) Further, when focusing has been effected by only the teleconverter, the range of focusing from infinity to a close distance is narrow and fluctuations of the aberrations are relatively great.